1. Field of Invention
The present invention relates to method and apparatus for concentrating and harvesting solar energy.
2. Brief Description of the Prior Art
This section is divided into three subsections. The prior art related to concentration of solar energy is reviewed in the first subsection and the prior art related to harvesting solar energy, concentrated and unconcentrated, is reviewed in the second subsection. And finally, the prior art related to storage of heat energy is reviewed in the third subsection.
2-A Concentration of Solar Energy
Conventionally, parabolic mirrors and Fresnel lenses have been used in concentrating Sun's rays. One may use a mirror in the shape of a paraboloid of revolution or a longitudinal mirror with parabolic cross-section taken in the direction perpendicular to the longitudinal direction of the mirror. The former may be simply referred to as a paraboloidal mirror and the latter may be referred to as a longitudinal parabolic mirror. Paraboloidal solar energy concentrators have been built by French Solar Laboratory in Pyreness, France, National Physical Laboratory in New Delhi, India (as solar cookers), and more recently by Westinghouse Corp., and Sunstrand Aviation Co., Denver, Colo. Longitudinal parabolic solar energy concentrators were built in Meadi, Egypt in 1913, later by Dr. Abott in U.S.A. and more recently by Drs. Aden and Marjorie Meinel in University of Arizona. Fresnel lenses can also be classified into circular Fresnel lenses and Longitudinal Fresnel lenses. C. P. Gilmore described a solar farm proposed by Drs. Meinel in Popular Science, Pp. 86-89, Dec. 1972. In this future solar farm, Drs. Meinel plan to use longitudinal Fresnel lenses for concentrating the Sun's rays on heat-collecting tubes. Circular Fresnel lenses and paraboloidal mirrors are obtainable from Edmund Scientific Co. N.J..
2-b Harvesting of Solar Energy
Solar energy may be harvested with or without a concentration step. In the solar heated houses that have been built, solar energy are harvested without a concentration step. In the famous Dover House deseigned by Dr. Maria Telkes, the collector or the harvestor is simply a black sheet of thin-gage iron mounted vertically behind double glass panes along the whole south wall on the first floor of the house. A low cost solar energy harvesting panel which comprises of a metal plate and multiplicity of beer cans coated black inside has recently been presented.
The solar power collector built by Drs. Meinel in the University of Arizona comprises of a longitudinal parabolic mirror, a glass vacuum pipe silvered inside and a coated steel pipe located within the glass vacuum pipe. A detail description of this unit appears in Popular Science, Pp. 86-89 and Pp. 112-114, Dec. 1972. The solar power collector operates as the parabolic mirror concentrates the Sun's rays on the coated steel tube. The coated steel tube is covered with thin-film optical layers to increase its heat-absorbing efficiency. Glass vacuum pipe is silvered inside except for small clear windows at bottom, which admit the concentrated sun's rays. Inside, light energy bounces around until it hits the coated steel pipe and is absorbed. The glass pipe is evacuated to prevent heat loss by convection. As the collector absorbs light in the visible range and converts it to heat, it radiates the heat in the form of infrared rays. Due to this radiative heat loss and other convective heat losses, a collector soon reaches a steady state temperature. Interference stacks have been used by this group of researchers to increase a/e ratio, where a is absorbance and e is emittance. Interference stacks are made by vacuum depositing thin layers of aluminum oxide, silicon or other substances on metal plates. The coated layer on the steel pipe form an interference stack.
Solar energy can also be harvested by a direct conversion into electric energy. Certain semiconductor materials such as silicon absorb photons of light and generate a small electric potential. The arrays of solar cells that power many spacecrafts work this way.
2-c Storing of Heat Energy
Solar energy harvested as heat energy can be stored as sensible heat of water and rocks or as latent heat of some selected substances. In the solar heated houses that have been built, the solar energy is converted into heat energy at a relatively low temperature level, 80.degree. to 125.degree.F, and the chemicals chosen for heat storage together with their melting points and heat of fusion are listed in Table 1.
TABLE 1 ______________________________________ Chemicals for Heat Storage ______________________________________ Melting Point, Heat of Fusion, Salt degrees F Btu per lb ______________________________________ Na.sub.2 CO.sub.3 10 H.sub.2 O 90-97 115 Na.sub.2 HPO.sub.4 12 H.sub.2 O 97-118 114 Na.sub.2 SO.sub.4 10 H.sub.2 O 88-90 104 Ca(NO.sub.3).sub.2 4 H.sub.2 O 104-108 90 Na.sub.2 S.sub.2 O.sub.3 5 H.sub.2 O 120-126 90 CaCl.sub.2 6 H.sub.2 O 84-102 75 ______________________________________ *The heat storage capacity of the above materials is about 10,000 Btu per cubic foot.